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102 lines
3.1 KiB
ReStructuredText
102 lines
3.1 KiB
ReStructuredText
==================================================
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How To Add A Constrained Floating-Point Intrinsic
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==================================================
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.. contents::
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:local:
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.. warning::
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This is a work in progress.
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Add the intrinsic
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=================
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Multiple files need to be updated when adding a new constrained intrinsic.
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Add the new intrinsic to the table of intrinsics::
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include/llvm/IR/Intrinsics.td
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Add SelectionDAG node types
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===========================
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Add the new ``STRICT`` version of the node type to the ``ISD::NodeType`` enum::
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include/llvm/CodeGen/ISDOpcodes.h
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Strict version name must be a concatenation of prefix ``STRICT_`` and the name
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of the corresponding non-strict node name. For instance, strict version of the
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node ``FADD`` must be ``STRICT_FADD``.
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Update mappings
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===============
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Add new record to the mapping of instructions to constrained intrinsics and
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DAG nodes::
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include/llvm/IR/ConstrainedOps.def
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Follow instructions provided in this file.
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Update IR components
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====================
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Update the IR verifier::
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lib/IR/Verifier.cpp
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Update Selector components
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==========================
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Building the SelectionDAG
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-------------------------
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The ``SelectionDAGBuilder::visitConstrainedFPIntrinsic`` function builds DAG nodes
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using mappings specified in ``ConstrainedOps.def``. If however this default build is
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not sufficient, the build can be modified, see how it is implemented for
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``STRICT_FP_ROUND``. The new ``STRICT`` node will eventually be converted
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to the matching non-``STRICT`` node. For this reason it should have the same
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operands and values as the non-``STRICT`` version but should also use the chain.
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This makes subsequent sharing of code for ``STRICT`` and non-``STRICT`` code paths
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easier::
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lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
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Most of the ``STRICT`` nodes get legalized the same as their matching non-``STRICT``
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counterparts. A new ``STRICT`` node with this property must get added to the
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switch in ``SelectionDAGLegalize::LegalizeOp()``::
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lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
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Other parts of the legalizer may need to be updated as well. Look for
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places where the non-``STRICT`` counterpart is legalized and update as needed.
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Be careful of the chain since ``STRICT`` nodes use it but their counterparts
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often don't.
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The conversion or mutation of the ``STRICT`` node to a non-``STRICT``
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version of the node happens in ``SelectionDAG::mutateStrictFPToFP()``. In most cases
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the function can do the conversion using information from ConstrainedOps.def. Be
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careful updating this function since some nodes have the same return type
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as their input operand, but some are different. Both of these cases must
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be properly handled::
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lib/CodeGen/SelectionDAG/SelectionDAG.cpp
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Whether the mutation happens or not depends on how the new node has been
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registered in ``TargetLoweringBase::initActions()``. By default, all strict nodes are
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registered with Expand action::
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lib/CodeGen/TargetLoweringBase.cpp
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To make debug logs readable, it is helpful to update the SelectionDAG's
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debug logger:::
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lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp
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Add documentation and tests
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===========================
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::
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docs/LangRef.rst
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